/* Try to generate the shortest string that will open up an electronic
   keypad, assuming that digits can be entered in the middle of a stream.
   example:  If the door unlock code is 1701
             then 12341701496 must work to unlock the door.
   If the lock requires hitting # or other special key after the code,
   then it breaks.
 
  (Please excuse the horrid C.  Its been a while since I've worked in C and
  I kind of did this as a refresher.)
  
   Code released into public domain.
*/


#include <stdio.h>
#include <strings.h>
#include <stdlib.h>
#include <time.h>

#define TRUE  1
#define FALSE 0
#define BOOL int


/* Set things up for a four digit code */
#define LASTNUM 9999
#define PLACES  4
#define FORMAT "%04d"



#define MAXLEN ( (PLACES) * (LASTNUM + 1) +1)




#define RANDOM FALSE  /* Randomize the numbers each time? */
/* Number of passes per run.  If RANDOM is false, then should be 1 */
#define PASSES 1

/* Quick and dirty version of malloc.  Init string. */
char *m_str(size_t size)
{
  char *d;
  d = malloc(size);
  if (d == NULL)
  {
    printf("Get rid of the VIC 20.  Memory low.\n");
    exit(1);
  }
  d[0] = 0;
  return(d); 
}


/* Trim down any digit that appears more than [count] times.
* For example: 0800 and 0001 would result in  08000001. 
* This can be safely turned into 080001
*/
char *strip(char *s, int count)
{
  int j;
  char *d; /* Destination */
 
  /* Named after x86 assembly language registers */
  long si; /* Src index */
  long di; /* Dest index */
  BOOL is_ok;
 
  d = m_str(MAXLEN +1);
 
  di = 0;
  si = 0;
  while (s[si] != 0) /* Loop to end of string */
  {
    j = 0;
    is_ok = FALSE;
    while (j < count)
    {
      /* If the character does NOT match the character that comes next */
      is_ok = ( (s[si + j] != s[si + j+1]) || is_ok);
      j++;
    }
   
    if (is_ok) {
      d[di] = s[si];
      di++;
    }
   
    si++;
  }
 
  d[di] = 0;
  return d;
}


void tell_me(char *s, int i)
{
  printf("Result is: %s  [Pass %4d] \n",s, i);
  printf("Length:%7d  Pass %4d\n",strlen(s), i);
  printf("\n");
}


char* doit(void)
{
  int i;
  char *loc;
  char *n;
  int where;
 
  char *list[LASTNUM+1];
  char *compact;
 
 
  compact = m_str(MAXLEN);
  srand( (unsigned int)time( NULL ) ); 
 
  /* First create the array */
  /* printf("Creating array\n"); */
  for (i = 0; i < LASTNUM + 1; i++)
  {
    n = m_str(PLACES+1);
    sprintf(n, FORMAT, i);
    list[i] = n;
  } 
 
  #if RANDOM
  /* Randomize the array */
  /* printf("Randomizing array\n"); */
  for (i = 0; i < LASTNUM + 1; i++)
  {
         where = rand() % LASTNUM;
    n = list[where];
    list[where] = list[i];
    list[i] = n;
  }
  #endif
 
  /* Build the string */
  /* printf("Building string\n"); */
  for (i = 0; i < LASTNUM + 1; i++)
  {
    n = list[i];
    loc = strstr(compact, n);
    if (loc == NULL) { /* Not found */
      strcat(compact, n);
    }
  }
 
  loc = strip(compact, PLACES);
 
  for (i = 0; i < LASTNUM + 1; i++)
  {
    free(list[i]);
  } 
 
  free(compact);
  return loc;
}

int main(void)
{
  int i;
  char *r;
 
  for (i = 0; i < PASSES; i++)
  {
    r = doit();
    tell_me(r, i);
    free(r);   
  }
  return(0);
}

Output, folded for readability.   If PASSES > 1 then the output is very grep and sort friendly.